1. Field of the Invention
This invention relates generally to integrated circuit memories and, more particularly, to a nonvolatile memory cell for use in integrated circuits.
2. Description of the Related Prior Art
Recently, materials displaying the ability to be reversibly switched between two stable electrical resistance states have been proposed for use in nonvolatile memories. When incorporated in a memory cell, these materials can be toggled between higher and lower electrical resistance states by applying a pulse of electrical current (“switching current pulse”to the materials. Subsequently, after writing to a memory cell in this way, the electrical resistance state of the given memory cell can be determined (i.e., read) by applying a sensing current pulse to the material in order to determine its electrical resistance state. The amplitude of the sensing current pulse is preferably sufficiently smaller than the amplitude of the switching current pulse so that the electrical resistance state of the material is not altered by the read operation, and the written electrical resistance state persists.
The electrical resistance of resistance-switching materials, including but not limited to transition-metal oxides and metal sulphides, can be changed significantly by external influences, including electrical fields, magnetic fields, and temperature. Electrical impulses applied to these materials can “program” memory devices, such that they exhibit a desired resistive property. Specifically, the referenced articles and U.S. Patents, all of which are hereby expressly incorporated by reference into the present invention for purposes including, but not limited to, indicating the background of the present invention and illustrating the state of the art, describe materials and classes of materials with programmable electrical resistance and simple electrical resistor devices made from these materials: “Reproducible switching effect in thin oxide films for memory applications” (A. Beck et at., Applied Physics Letters, Vol. 77, No. 1, July 2000); “Quantized conductance atomic switch” (K. Terabe et at., Nature 433, 6 Jan. 2005); U.S. Pat. No. 6,815,744 issued to Beck et al on Nov. 9, 2004; and, U.S. Pat. No. 6,204,139 issued to Liu et al. on Mar. 20, 2001.
Transition-metal oxides and metal sulphides are classes of materials that can be conditioned such that they exhibit the desired bistable electrical resistance. This conditioning process involves subjecting the insulating dielectric material to an appropriate electrical signal for a sufficient period of time. This conditioning process is believed to generate a confined conductive region of arbitrary shape in the transition-metal oxide or metal sulphide. This region is formed near local perturbations such as vacancies, defects, impurities, grain boundaries, or roughness. The conditioning process of the programmable resistance materials can be accelerated, for example, by incorporating oxygen vacancies in the material during the fabrication process. It is believed that it is the interface region near the electrodes that can be reversibly switched between two or more resistance states by applying a pulse of electrical current to the materials.
The confined conductive region is generated at an arbitrary position in the dielectric material, i.e., the position of the conducting path is not controlled by well-defined process parameters. This leads to a large variation in the electrical properties of nominally identical memory cells comprising conventional programmable resistors and of devices comprising such memory cells. Moreover, only a part of the area defined by the electrodes is used for current flow. Hence, the confined region is subject to local heating and to potential damage.
The time-consuming conditioning process and the large variations of the properties of nominally identical programmable resistors used in the memory cells and in devices formed with such memory cells severely hinders manufacturability and is impractical for production type arrays.
A disadvantage of the above-mentioned material is that a conditioning process is required.
Another disadvantage of the prior-art memory devices is their long response time, ranging from typically 100 ns to 10 μs.
A further disadvantage is the large statistical spread of the operating conditions of the devices.